1. Field of the Invention
The present invention relates to ceramic electronic components, and, more particularly, to a ceramic electronic component including a ceramic body, two-layered terminal electrodes, lead terminals and solder.
2. Description of the Related Art
A conventional ceramic electronic component of such a type will be described with reference to FIGS. 1A and 1B. As shown in the drawing, a ceramic electronic component 11 includes a ceramic body 12 mainly composed of a dielectric material, insulator, semiconductor, piezoelectric material, magnetic material or the like; terminal electrodes 13 formed on both principal surfaces of the ceramic body 12; lead terminals 15 joined to the corresponding terminal electrodes 13 with solder 14; and an outer resin section 16 which is formed so as to cover the ceramic body 12, the terminal electrodes 13, the solder 14 and parts of the lead terminals 15.
The terminal electrodes 13 include thin films or thick films formed by sputtering or vapor deposition and plating films containing, for example, a noble metal, such as Ag or Pd, or an alloy thereof, or a base metal, such as Ni or Cu, or an alloy thereof, as a conductive constituent. As the solder 14, an alloy containing Sn/Pb as a principal constituent is commonly used. The lead terminal 15, for example, includes a core composed of Cu, Fe or the like, which is coated by Sn-Pb or Sn plating.
However, when the ceramic electronic component in accordance with the conventional technique is exposed to a high temperature environment for a long period of time, intermetallic compounds are formed at the joint interfaces between the terminal electrode and the solder and between the terminal electrode and the ceramic body due to interdiffusion between Sn contained in the solder and the conductive constituent of the terminal electrode mainly composed of Ag, Cu, Ni or the like. Since the intermetallic compound is rigid and brittle, bonding reliability is decreased and separation occurs between the ceramic body and the terminal electrode when the entire terminal electrode is transformed into the intermetallic compound, resulting in degradation in electrical characteristics.
When Sn/Pb solder is used, a Pb-rich phase is generated at the interface between the intermetallic compound and the solder due to the diffusion of Sn. If the ceramic electronic component has a structure in which the soft Pb-rich phase and the rigid, brittle intermetallic compound are brought into contact with each other, cracking occurs in the terminal electrode, etc., when stress is applied, resulting in degradation in bonding reliability.
It is an object of the present invention to provide a ceramic electronic component in which stable electrical characteristics are ensured even if exposed to a high temperature environment for a long period of time and which has satisfactory bonding strength between the ceramic body and terminal electrodes and between the terminal electrodes and lead terminals.
In one aspect of the present invention, a ceramic electronic component includes a ceramic body, terminal electrodes formed on the ceramic body, and lead terminals joined to the terminal electrodes with solder containing Sn. Each terminal electrode includes a first electrode layer formed on the ceramic body and a second electrode layer formed on the first electrode layer, and the second electrode layer contains a conductive constituent containing at least Zn, Ag and/or Cu, and Sn. The Zn content in the second electrode layer is about 4% by weight or more in relation to 100% by weight of the conductive constituent, and is within the solubility limit so that it does not form AgZn and/or CuZn intermetallic compounds.
In the ceramic electronic component in which the lead terminal is joined to the second electrode layer with the solder, AgZn and/or CuZn intermetallic compounds must be prevented from being formed after soldering in order to achieve the object of the present invention. Incidentally, the solubility limit for not forming AgZn and/or CuZn intermetallic compounds are not determined by the Zn content directly and exclusively. It is determined by the cooling rate from the molten state to the solid state and the solid-state temperature after cooling. In particular, when soldering is performed as in the case of the present invention, since the cooling rate is fast, the nonequilibrium state is brought about, and it is not possible to numerically express the solubility limit using the Zn content. Therefore, the Zn content is defined as being within the solubility limit which does not form AgZn and/or CuZn intermetallic compounds.
In another aspect of the present invention, a ceramic electronic component includes a ceramic body, terminal electrodes formed on the ceramic body, and lead terminals joined to the terminal electrodes with solder containing Sn. Each terminal electrode includes a first electrode layer formed on the ceramic body and a second electrode layer formed on the first electrode layer, and a barrier layer is formed in the first electrode layer and/or the second electrode layer due to the flow and concentration of Zn in the second electrode layer.
Preferably, the barrier layer lies in the first electrode layer and is formed within a range of about 2 xcexcm from the interface with the ceramic body.
Preferably, the first electrode layer is composed of a thin film or a thick film and contains a conductive constituent containing at least Ag and/or Cu.
Preferably, the lead terminals are coated with an alloy which does not contain Pb apart from incidental impurities or a metal other than Pb, and the solder does not contain Pb apart from incidental impurities.
Preferably, the second electrode layer is formed by hot dipping.
A ceramic electronic component of the present invention includes two-layered terminal electrodes, each including a first electrode layer formed on a ceramic body and a second electrode layer formed on the first electrode layer. The second electrode layer contains a conductive constituent containing at least Zn, Ag and/or Cu, and Sn.
When the ceramic electronic component is subjected to thermal aging after the second electrode layer is provided on the ceramic electronic component, Zn in the second electrode layer is concentrated in the vicinity of the interface with the first electrode layer to form a barrier layer. The barrier layer flows into the first electrode layer as thermal aging advances, and then reaches the vicinity of the interface between the first electrode layer and the ceramic body. Since the barrier layer suppresses interdiffusion between Sn contained in the solder formed on the second electrode layer and the parent metal in the first electrode layer, intermetallic compounds are inhibited from being formed at the interface between the second electrode layer and the solder and at the interface between the ceramic body and the first electrode layer, and thus separation between the terminal electrode and the solder and separation between the ceramic body and the terminal electrode are prevented. Therefore, stable electrical characteristics are ensured even if exposed to a high temperature environment for a long period of time and satisfactory bonding strength is obtained between the ceramic body and the terminal electrodes and between the terminal electrodes and the lead terminals. Additionally, the barrier layer is not necessarily required to be formed in the entire region or to the same level in the second electrode layer or the first electrode layer, and the barrier layer may be formed substantially as a film of varying thickness so that interdiffusion with the parent metal in the first electrode layer is inhibited.
The Zn content in the second electrode layer must be about 4% by weight or more in relation to 100% by weight of the conductive constituent constituting the second electrode layer, and must be within the solubility limit not forming AgZn and/or CuZn intermetallic compounds. If the Zn content is about 4% by weight or more, a barrier layer is formed in the second electrode layer or the first electrode layer, and interdiffusion between Sn in the solder and the parent metal in the first electrode layer is inhibited. That is, intermetallic compounds are inhibited from being formed at the interface between the second electrode layer and the solder and at the interface between the ceramic body and the first electrode layer, resulting in an improvement in bonding reliability between the ceramic body and the terminal electrode and between the terminal electrode and the solder. If the Zn content is within the solubility limit in which brittle AgZn and/or CuZn intermetallic compounds in the xcex2, xcex3, and xcex5 phases shown in Hansen""s binary phase diagram are not formed, bonding reliability between the ceramic body and the terminal electrode and between the terminal electrode and the solder is maintained since intermetallic compounds are inhibited from being formed at the interface between the second electrode layer and the solder and at the interface between the ceramic body and the first electrode layer. Therefore, stable electrical characteristics are ensured even if exposed to a high temperature environment for a long period of time and satisfactory bonding strength is obtained between the ceramic body and the terminal electrodes and between the terminal electrodes and the lead terminals. In general, since Zn is a base metal, it is easily subjected to electrolytic etching, resulting in a decrease in the mechanical strength of the terminal electrode. However, if the Zn content is within the ranges described above, such a problem is not encountered.
The barrier layer preferably lies in the first electrode layer and is formed within a range of about 2 xcexcm from the interface with the ceramic body. As described above, the concentrated Zn in the second electrode layer flows into the first electrode layer as thermal aging advances and then reaches the vicinity of the interface between the first electrode layer and the ceramic body. However, it does not easily diffuse into the ceramic body. Therefore, if the barrier layer formed due to the concentration of Zn lies in the first electrode layer and in the vicinity of the interface with the ceramic body, Zn which flows therein as thermal aging advances accumulates. In such a case, a barrier layer formed within about 2 xcexcm from the interface with the ceramic body has the most uniform thickness, which is advantageous.
The lead terminals are preferably coated with an alloy which does not contain Pb apart from incidental impurities or a metal other than Pb, and the solder preferably does not contain Pb apart from incidental impurities. If Pb is not present in the lead terminals and the solder, a Pb-rich phase is not easily produced even if Sn in the solder is diffused, and therefore, even if stress is applied to the terminal electrodes, cracking does not easily occur, resulting in an increase in bonding reliability.